Crystallization method and crystallization apparatus

ABSTRACT

A crystallization method includes the steps of melting a crystallized material in a crucible by heating, and growing a crystal by cooling and coagulating the melted material, wherein said melting step includes introducing a predetermined gas into the melted material.

BACKGROUND OF THE INVENTION

The present invention relates mainly to a method and apparatus thatgrows a crystal by a solidification process that solidifies meltedcrystal material.

Various existing methods to produce an excellent single crystal bycontrolling a solidification process that solidifies melted crystalmaterial are conventional used for manufacture various crystalmaterials.

Typical manufacturing method for the single crystal is the CZ(Czochralski) method that slowly lifts a solid crystal from solidifyingmelted material in a crucible and the Bridgman method that is solidifiesthe melted material in one direction by changing a temperaturedistribution in the crucible that houses the melted material. The choiceof single crystal manufacturing method depends on requiredcharacteristics and performance from the manufactured single crystal.

The single crystal manufactured by the above crystallization method is,generally, high-purity. Moreover, it is high demand for the singlecrystal not to include defects such as air bubbles. Then, the crystal isgrown at a slow crystal growth rate by using a melt liquid consisting ofan enough high-purity crystal material so that the impurities and airbubbles are not included in the manufactured single crystal.

More particularly, when calcium fluoride is used as an optical elementfor ultraviolet etc. with short wavelength, an optical performance isgreatly deteriorated because of extremely small amount of impurities, soa scavenger to remove impurities is added to the melt liquid during thecrystal growth process, and high-purity single crystal is manufactured.For instance, refer to “Single Crystal Growth Technology (Tsuguo Fukuda,Keigo Hoshikawa., BAIFUKAN CO., LTD)”.

However, in actual crystal growth process, impurities and air bubblesthat mix into the grown single crystal cannot always be completelyremoved. This is because impurities remain such as the solid crystalmaterial before melt, gas molecules that adhere to the crucible surface,and adhered particles of high melting point that can not desorbed fromthe melt liquid when viscosity is comparatively high.

Moreover, it is necessary to remove a reaction product to outside ofsystem by adding a high-purity scavenger as a gas for the above calciumfluoride. However, all the gas cannot be exhausted, and remains in thegrown crystal for the above reasons.

The present invention is invented to solve the above problem, andprovides crystallization method and crystallization apparatus thatefficiently removes gas and solid particles of high melting point thatexist in a melt liquid during the crystal growth process to outside ofsystem, and grows high-purity crystal that does not include air bubbles.

BRIEF SUMMARY OF THE INVENTION

In order to achieve the above object, a crystallization method accordingto one aspect of the present invention that includes the steps ofmelting a crystallized material in a crucible by heating; and growing acrystal by cooling and coagulating the melted material, wherein saidmelting step includes introducing a predetermined gas into the meltedmaterial.

An optical element according to another aspect of the present inventionmade of a single crystal, said single crystal being manufactured by acrystallization method that includes melting a crystallized material ina crucible by heating, and growing a crystal by cooling and coagulatingthe melted material, wherein said melting step includes introducing apredetermined gas into the melted material, and wherein saidcrystallized material is a calcium fluoride with an added scavenger, andthe gas introduced into the melted fluorite is an inert gas.

An exposure apparatus according to another aspect of the presentinvention includes a light source, an illumination optical system forguiding a light from the light source to a reticle, and a projectionoptical system for guiding the light from the reticle to a wafer that isplaced on a wafer stage, wherein said illumination optical system or theprojection optical system includes an optical element, and wherein saidoptical element is made of a single crystal, said single crystal beingmanufactured by a crystallization method that includes, melting acrystallized material in a crucible by heating, and growing a crystal bycooling and coagulating the melted material, wherein said melting stepincludes introducing a predetermined gas into the melted material, andwherein said crystallized material is a calcium fluoride with an addedscavenger, and the gas introduced into the melted calcium fluoride is aninert gas.

A crystallization apparatus according to another aspect of the presentinvention includes a crucible arranged in a predetermined temperaturedistribution, which houses a melted crystallized material, and a gasintroducing part for introducing a gas into the melted crystallizedmaterial, wherein the crystallization apparatus coagulates the meltedcrystallized material in one direction by changing the temperaturedistribution relatively to the crucible.

A crystallization apparatus according to another aspect of the presentinvention includes a crucible heated at a predetermined temperature, andhouses a melted crystallized material, and a gas introducing part forintroducing a gas into the melted crystallized material, wherein thecrystallization apparatus grows a crystal by lifting a member in contactwith the melted crystallized material.

The present invention can produce a crystal with low concentration ofimpurities and does not include air bubbles etc. by introducing a gasinto a melt liquid used for the crystal growth and shaking.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a typical sectional view of a crystallization apparatusaccording to the present invention.

FIG. 2 is typical view of state that particles and gas impurities in amelt liquid are removed by introducing a gas.

FIG. 3 is a view of a relationship between an introducing time thathelium gas is introduced into a melt liquid of calcium fluoride beforethe crystal growth begins and a density of melt liquid.

FIG. 4 is a flowchart that shows a process flow from a materialsynthesis of calcium fluoride single crystal that grows in the presentinvention to an exposure apparatus.

FIG. 5 is schematic sectional view of an exposure apparatus using thecalcium fluoride single crystal grown by the present invention.

FIG. 6 is a flowchart for explaining how to fabricate devices (such assemiconductor chips such as ICs, LCDs, CCDs, and the like).

FIG. 7 is a detailed flowchart of a wafer process in Step 4 of FIG. 6.

FIG. 8 is a typical sectional view of a second crystallization apparatusaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a typical sectional view of a crystallization apparatusaccording to the present invention. In FIG. 1, a side heater 3 a made ofgraphitized carbon with cylindrical form is arranged in a housing 5 thatforms a chamber 6. The side heater 3 a is supplied an electric powerfrom a side heater power source 7 that is controlled by a controller 9,and forms a predetermined temperature distribution in the chamber 6. Thetemperature of the chamber 6 is measured by the temperature sensor 18,and feedback to the controller 9. An insulator 4 made of graphitizedcarbon is installed inside the housing 5, and protects the housing 5from the high-temperature. The chamber 6 is vacuum exhausted to thepressure of 1E-4 or less (Torr) by the exhaust apparatus (not shown).

A crucible support rod 2 that supports a crucible 1 is installed topenetrate through a bottom part of the housing 5. The crucible 1 housesa material as a crystalloid. A crucible lifting motor 2 a drives thecrucible support rod 2 in a vertical direction at a predetermined speedby an electric power from a crucible lifting motor power source 2 b thatis controlled by the controller 9. A bottom part of the chamber 6 ismaintained at the temperature below a melting point of the growncrystal. The crystal coagulates from an upper side in the crucible 1 inone direction, and grows by lowering down the crucible 1 by the cruciblesupport rod 2.

The crystallization apparatus according to the present invention furtherincludes a gas introducing port 11 that introduces a gas into thecrucible 1. The gas introducing port 11 is supplied a predetermined gasfrom a gas tank 16 through a gas valve 14, a valve controller 17, a gaspurification apparatus 13, and a pipe 12 etc. The gas introducing port11 has a movable (up and down) structure to insert into the melt liquidin the crucible 1 to introduce the gas after the material as thecrystalloid is melted, and to shelter it from the crucible 1 during thecrystal growth. The gas introducing port 11 is made of a same materialas crucible 1 so as not to pollute the melt liquid.

FIG. 2 is typical view of state that particles and gas impurities in themelt liquid are removed by introducing the gas. FIG. 2 shows the casewhere helium is introduced into the calcium fluoride, but the presentinvention is not limited to this. For instance, generally, the effect ofthe present invention can be achieved by introducing the gas into themelt liquid used for the crystal growth.

When the air bubbles and particle exist in the melt liquid, the buoyancyF that acts on the air bubbles etc. is shown by a reaction formula (1).F=(ρ−ρ′)gV  (1)

Here, ρ is a density of the melt liquid, ρ′ is a density of the airbubbles etc., g is a gravitational acceleration, and V is a volume ofthe air bubbles. When the air bubbles exists in the melt liquid,generally, ρ′ can be disregarded for ρ because the density of the airbubbles is small enough compared with the density of the melt liquid.

Originally, the air bubbles existed at the melt liquid surface, and isexhausted to the outside of the system. Because the buoyancy isproportional to the volume of the air bubbles as understood from thereaction formula (1), the buoyancy is much smaller for air bubbles thathave a diameter of plural tens μm or less (micro-bubble) which causesproblems during crystal growth. Therefore, the air bubble cannotsurface, and is present in the grown crystal. Especially, when thedensity of the air bubbles included in the melt liquid is belowconstancy by a constant management, there is little thing to which thebubble's uniting increases the volume, and the air bubbles of a certainamount remains even if it dissolves for a long time.

Moreover, when the air bubbles etc. that exist in the melt liquid reactswith the melt liquid, a secondary reaction is caused because the airbubbles exist in the melt liquid for a long time and becomes impuritiesin the melt liquid.

As shown in FIG. 2, the micro-bubbles that cannot surface in the meltliquid can be effectively exhausted to the outside of the system byintroducing the predetermined gas as a bubble that has a diameter morethan certainly into the melt liquid. In other words, when an enough bigbubbles surface in the melt liquid, it is possible to exhaust the airbubbles to the outside of the system by catching the micro-bubbles. Whena minute particle as solid exists in the melt liquid, the particle istaken into the bubble by a surface tension between the particle and themelt liquid, and can be exhausted to the outside of the system. Theparticle carried to the surface of the melt liquid once by the bubbledoes not mix again into the melt liquid by the surface tension.

It is desirable that the gas introduced into the melt liquid does notbecome a harmful impurity in the melt liquid, and is promptly exhaustedto outside of the melt liquid. As explained by the followingembodiments, an inert gas that does not generate impurities is desirablefor the melt liquid of the calcium fluoride, and helium is moredesirable because the density is low and the diffusion speed is high inthe melt liquid. On the other hand, it is desirable for the melt liquidof the oxide crystal such as niobic acid lithium and the tantalic acidlithium etc. to introduce oxygen for the maintenance of stoichiometry.The acceptable inert gas is similar to the calcium fluoride, and heliumis more desirable because the density is low and the diffusion speed ishigh in the melt liquid.

The gas introduced is of a high purity to not introduce impurities alongwith the introduction of the gas.

First Embodiment

The instant embodiment explains an example of manufacturing the calciumfluoride chiefly used for an optical material by the abovecrystallization method.

A material that adds ZnF₂ of 0.1% by weight as the scavenger tohigh-purity calcium fluoride polycrystal which is a material of calciumfluoride single crystal is placed into the carbon crucible 1 shown inFIG. 1. After the chamber 6 is vacuum-exhausted, the side heater 3 iselectrified to heat the chamber 6, the crucible 1 is adjusted to about1360° C., and the material in the crucible 1 is melted. The melt stateis maintained for about three hours for the oxygen removal reaction tooccur in the melt liquid by the scavenger, and then, the gas introducingport 11 made of carbon is inserted in the crucible 1, and the state ismaintained for seven hours while introducing the helium gas with 300sccm. Meanwhile, the chamber 6 is maintained to the pressure of 1 Torror less to easily deaerate the introduced helium gas.

Then, the introduction of the helium gas is ended, and the gasintroducing port 11 is taken out of the crucible 1, and the melt stateis maintained for about two hours to deaerate the introduced helium gas.After the vacuum level of the chamber 6 is 2E-6 Torr or more, thecrucible 1 is descended at the speed of 1 mm/h so as to grow the calciumfluoride single crystal.

Table 1 shows a residual concentration of Zn (analyzed with ICP) anddensity of the micro-bubbles that has diameter of 30 μm or less (numberfor each 1 liter crystal) of the calcium fluoride crystal grown by theinstant embodiment. Table 1 shows a residual concentration of Zn anddensity of micro-bubbles of a calcium fluoride crystal that occur in thecrystal after the melt state is maintained for twelve hours withoutintroducing the helium gas for the comparison. TABLE 1 CALCIUM FLUORIDEOF THE INSTANT COMPARISON EMBODIMENT EXAMPLE RESIDUAL CONCENTRATION 5 57OF Zn (ppb) DENSITY OF MICRO-BUBBLE 3.1 32.7 (number/L)

Reference to Table 1, the residual concentration of Zn and density ofmicro-bubbles have decreased when the helium gas is introduced (theinstant embodiment). It is thought that this depends on the followingactions:

The decrease in the residual concentration of Zn by the introduction ofthe helium gas is thought to be a result of the removal by theevaporation of the zinc included in the added scavenger by the gasintroduction. The added ZnF₂ as the scavenger removes oxygen in the meltliquid of calcium fluoride by the reaction shown in the following areaction formulas (2) and (3).CaO+ZnF₂→CaF₂+ZnO  (2)ZnO+C (crucible etc.)→Zn↑+CO↑  (3)

As shown in the reaction formula 3, the metallic zinc generated by thereaction with the scavenger evaporates and is excluded from the meltliquid because the vapor pressure is high. However, the concentration ofzinc is actually low, and the bubble of size that obtains the enoughbuoyancy to desorb from the melt liquid can not be formed. Therefore,the desorption from the melt liquid is limited to the desorption fromthe melt liquid surface.

On the other hand, when the helium gas is introduced (the instantembodiment), the micro-bubble of the zinc (or vacuole) that exists inthe melt liquid contacts the helium gas, is taken into the helium gas asvapor, and is exhausted to the outside of the melt liquid.

The decrease in the micro-bubbles by the introduction of the helium gasis understood as follows: The generation cause of the micro-bubble isnot clear. However, it is thought that the micro-bubble is caused by thegas's that exist in the melt liquid, and the micro-bubble is caused bythe crystal growth's being locally obstructed.

For instance, CO etc. generated by the reaction with the scavenger donot have the enough buoyancy to desorb from the melt liquid because theyare generated as the micro-bubble in the melt liquid, and is taken intothe grown crystal easily. CO that exists in the melt liquid for a longtime as the micro-bubbles generates a minute amount of solid carbon byheat-resolving, and causes the micro-bubble of the crystal being presentin the crystal.

On the other hand, when the helium gas is introduced (the instantembodiment), the micro-bubble that exists in the melt liquid contactsthe helium gas, is taken into the helium gas as vapor, and is exhaustedto the outside of the melt liquid. When remaining as the air bubbles inthe melt liquid for certain time after mixing with the helium gas, areactive speed with the melt liquid etc. decreases due to the decreasein the partial pressure due to the mixing, and a generation of asecondary particles and a mixing of impurities can be prevented.

Moreover, a particle that mixes from a refractory material such ascrucible etc. is exhausted to the melt liquid surface by introducing thehelium gas. It is thought that the amount of micro-bubbles in the growncrystal decreases by these effects.

FIG. 3 is a view of a relationship between an introducing time thathelium gas is introduced into the melt liquid of calcium fluoride beforethe crystal growth and the density of the melt liquid. The density ismeasured by Archimedes method that uses a gauge head made of the carbonat the melting point of 1360° C. The density of the melt liquidincreases as the introducing time of the gas becomes long. The cause ofthe density changing of the melt liquid shown in FIG. 3 is not clear.However, it is thought that it is the result of the promotion of theexhaust of the air bubbles that exist in the melt liquid.

FIG. 4 is a flowchart that shows a process flow from a synthesis ofmaterial of calcium fluoride single crystal that grows in the presentinvention to a device assembly.

A high-purity calcium fluoride polycrystal above, used as material ofcalcium fluoride single crystal, is synthesized by the followingprocesses. First, a calcium carbonate and a hydrogen fluoride are madeto react as in a reaction formula (4), and a powdery calcium fluoride issynthesized.CaCO₃+2HF→CaF₂+H₂O+CO₂  (4)

In the purification process that bakes the calcium fluoride generated bythe reaction formula (4), the baked calcium fluoride mixes with thescavenger, and melts in the carbon crucible, for example, the oxygencontained in the calcium fluoride as a burnt lime is removed as shown inreaction formula (5).CaO+ZnF₂→CaF₂+ZnO↑  (5)

The scavenger preferably includes zinc fluoride, bismuth fluoride,sodium fluoride, lithium fluoride, and others which are more easilycombinable with oxygen mixed in the calcium fluoride than the calciumfluoride, and easily decomposes and evaporates. The zinc fluoride ispreferable. In the purification process, the scavenger is added by 0.05mol % to 5.0 mol %, desirably 0.1 mol % to 1.0 mol %. The calciumfluoride polycrystal obtained thus is used to manufacture the calciumfluoride single crystal.

The instant embodiment explained the gas introduction in the growthprocess of the calcium fluoride crystal, but even if the gas isintroduced in the above purification process, a similar effect can beachieved. Therefore, the purity of the material used for the crystalgrowth is improved. Moreover, it is applicable when scavengers otherthan the zinc fluoride are used.

In addition, the present invention is not limited to manufacturing thecalcium fluoride crystal. When the material in the crucible iscoagulated in one direction and the crystal is manufactured, the presentinvention can be applied.

Heat treatment processing is performed on the grown calcium fluoridesingle crystal in an anneal chamber. This process heats the calciumfluoride single crystal to 900° C. to 1300° C. in the crucible. Theheating time is 20 hours or more, desirably 20 hours to 30 hours, andthen the calcium fluoride single crystal is cooled at cooling speed ofabout 1° C./hour. Then, the calcium fluoride single crystal is formedinto a shape of a prescribed optical element, and used for an opticalsystem.

FIG. 5 is schematic sectional view of an exposure apparatus using thecalcium fluoride single crystal grown by the present invention.Referring now to FIG. 5, a description will be given of the exposureapparatus 500. The exposure apparatus 500 includes, as shown in FIG. 5,an illumination apparatus 510 for illuminating a reticle 520 which formsa circuit pattern, a projection optical system 530 that projectsdiffracted light created from the illuminated reticle pattern onto aplate 540, and a stage 545 for supporting the plate 540.

The exposure apparatus 500 is a projection exposure apparatus thatexposes onto the plate 540 a circuit pattern created on the reticle 520,e.g., in a step-and-repeat or a step-and-scan manner. Such an exposureapparatus is suitable for a sub-micron or quarter-micron lithographyprocess. This embodiment exemplarily describes as a step-and-scanexposure apparatus (which is also called “a scanner”). The“step-and-scan manner”, as used herein, is an exposure method thatexposes a reticle pattern onto a wafer by continuously scanning thewafer relative to the reticle, and by moving, after an exposure shot,the wafer stepwise to the next exposure area to be shot. The“step-and-repeat manner” is another mode of exposure method that moves awafer stepwise to an exposure area for the next shot, for every cellprojection shot.

The illumination apparatus 510 which illuminates the reticle 520 thatforms a circuit pattern to be transferred, includes a light source unit512 and an illumination optical system 514.

As an example, the light source unit 512 uses a light source such as ArFexcimer laser with a wavelength of approximately 193 [nm] and KrFexcimer laser with a wavelength of approximately 248 [nm]. However, thelaser type is not limited to excimer lasers because for example, F₂laser with a wavelength of approximately 157 [nm] and a YAG laser may beused. Similarly, the number of laser units is not limited. For example,two independently acting solid lasers would cause no coherence betweenthese solid lasers and significantly reduces speckles resulting from thecoherence. An optical system for reducing speckles may swing linearly orrotationally. When the light source unit 512 uses laser, it is desirableto employ a beam shaping optical system that shapes a parallel beam froma laser source to a desired beam shape, and an incoherently turningoptical system that turns a coherent laser beam into an incoherent one.A light source applicable for the light source unit 512 is not limitedto a laser, and may use one or more lamps such as a mercury lamp and axenon lamp.

The illumination optical system 514 is an optical system thatilluminates the reticle 520, and includes a lens, a mirror, a lightintegrator, a stop, and the like, for example, a condenser lens, afly-eye lens, an aperture stop, a condenser lens, a slit, and animage-forming optical system in this order. The illumination opticalsystem 514 can use any light regardless of whether it is axial ornon-axial light. The light integrator may include a fly-eye lens or anintegrator formed by stacking two sets of cylindrical lens array plates(or lenticular lenses), and can be replaced with an optical rod or adiffractive element. The inventive calcium fluoride crystal isapplicable to optical elements, such as, a lens in the illuminationoptical system 514.

The reticle 520 is made, for example, of quartz, forms a circuit pattern(or an image) to be transferred, and is supported and driven by a maskstage (not shown). Diffracted light emitted from the reticle 520 passesthrough the projection optical system 530 and is then projected onto theplate 540. The reticle 520 and the plate 540 are located in an opticallyconjugate relationship. Since the exposure apparatus 500 of thisembodiment is a scanner, the reticle 520 and the plate 540 are scannedat the speed ratio of the reduction ratio of the projection opticalsystem 530, thus transferring the pattern from the reticle 520 to theplate 540. If it is a step-and-repeat exposure apparatus (referred to asa “stepper”), the reticle 520 and the plate 540 remains still whenexposing the mask pattern.

The projection optical system 530 is an optical system that projectslight that reflects a pattern on the reticle 520 located on an objectsurface onto the plate 540 located on an image surface. The projectionoptical system 530 may use an optical system comprising solely of aplurality of lens elements, an optical system including a plurality oflens elements and at least one concave mirror (a catadioptric opticalsystem), an optical system including a plurality of lens elements and atleast one diffractive optical element such as a kinoform, a full mirrortype optical system, and so on. Any necessary correction of thechromatic aberration may be accomplished by using a plurality of lensunits made from glass materials having different dispersion values (Abbevalues) or arranging a diffractive optical element such that itdisperses light in a direction opposite to that of the lens unit. Anoptical element made of the inventive calcium fluoride crystal isapplicable to any optical element, such as a lens in the projectionoptical system 530.

The plate 540, such as a wafer and a LCD, is an exemplary object to beexposed. Photoresist is applied to the plate 540. A photoresistapplication step includes a pretreatment, an adhesion acceleratorapplication treatment, a photo-resist application treatment, and apre-bake treatment. The pretreatment includes cleaning, drying, etc. Theadhesion accelerator application treatment is a surface reformingprocess to enhance the adhesion between the photoresist and a base(i.e., a process to increase the hydrophobicity by applying a surfaceactive agent), through a coat or vaporous process using an organiccoating such as HMDS (Hexamethyl-disilazane). The pre-bake treatment isa baking (or burning) step, which makes the photoresist softer thanafter development and removes the solvent.

The stage 545 supports the plate 540. The stage 545 may use anystructure known in the art, thus, a detailed description of itsstructure and operation is omitted. The stage 545 may use, for example,a linear motor to move the plate 540 in the XY directions. The reticle520 and plate 540 are, for example, scanned synchronously, and thepositions of the stage 545 and a mask stage (not shown) are monitored,for example, by a laser interferometer and the like, so that both aredriven at a constant speed ratio.

The stage 545 is installed on a stage stool supported on the floor andthe like, for example, via a dampener. The mask stage and the projectionoptical system 530 are installed on a lens barrel stool (not shown)support, for example, via a dampener, to the base frame placed on thefloor.

In exposure, light is emitted from the light source 512, e.g.,Koehler-illuminates the reticle 520 via the illumination optical system514. Light that passes through the reticle 520 and reflects the maskpattern is imaged onto the plate 540 by the projection optical system530. The illumination and projection optical systems 514 and 530 in theexposure apparatus 500 include an optical element made of inventivecalcium fluoride crystal that transmits the UV light, FUV light, and VUVlight with high transmittance, and provide high-quality devices (such assemiconductor devices, LCD devices, photographing devices (such as CCDs,etc.), thin film magnetic heads, and the like) with high throughput andeconomic efficiency.

Referring now to FIGS. 6 and 7, a description will be given of anembodiment of a device fabrication method using the above mentionedexposure apparatus 500. FIG. 6 is a flowchart for explaining how tofabricate devices (i.e., semiconductor chips such as IC and LSI, LCDs,CCDs, and the like). Here, a description will be given of thefabrication of a semiconductor chip as an example. Step 1 (circuitdesign) designs a semiconductor device circuit. Step 2 (maskfabrication) forms a mask having a designed circuit pattern. Step 3(wafer making) manufactures a wafer using materials such as silicon.Step 4 (wafer process), which is also referred to as a pretreatment,forms the actual circuitry on the wafer through lithography using themask and wafer. Step 5 (assembly), which is also referred to as apost-treatment, forms into a semiconductor chip the wafer formed in Step4 and includes an assembly step (e.g., dicing, bonding), a packagingstep (chip sealing), and the like. Step 6 (inspection) performs varioustests on the semiconductor device made in Step 5, such as a validitytest and a durability test. Through these steps, a semiconductor deviceis finished and shipped (Step 7).

FIG. 7 is a detailed flowchart of the wafer process in Step 4. Step 11(oxidation) oxidizes the wafer's surface. Step 12 (CVD) forms aninsulating layer on the wafer's surface. Step 13 (electrode formation)forms electrodes on the wafer by vapor disposition and the like. Step 14(ion implantation) implants ion into the wafer. Step 15 (resist process)applies a photosensitive material onto the wafer. Step 16 (exposure)uses the exposure apparatus 500 to expose a circuit pattern from themask onto the wafer. Step 17 (development) develops the exposed wafer.Step 18 (etching) etches parts other than a developed resist image. Step19 (resist stripping) removes unused resist after etching. These stepsare repeated to form multi-layer circuit patterns on the wafer. Use ofthe fabrication method in this embodiment helps fabricate higher-qualitydevices than conventional methods. Thus, the device fabrication methodusing the exposure apparatus 500, and resultant devices constitute oneaspect of the present invention.

Second Embodiment

FIG. 8 is a typical sectional view of a second crystallization apparatusaccording to the present invention. FIG. 8 shows a structure when thisinvention is executed with the crystallization apparatus by Czochralskimethod. In the instant embodiment, a description will be given ofmanufacturing of lithium tantalate (LiTaO₃) (hereafter, LT) crystal asone example of the grown crystal.

In FIG. 8, a crucible 1 a that houses a melting material as acrystalloid is installed in the housing 5 that form the chamber 6. Aninsulator 4 is installed inside the housing 5, and protects the housing5 from the high-temperature. The chamber 6 is vacuum exhausted to thepressure of 1E-4 or less (Torr) by the exhaust apparatus (not shown).

A RF (radiofrequency generation) coil 21 that heats the material in thecrucible 1 a is arranged outside of the chamber 6. The RF coil 21 issupplied an electric power from a radiofrequency generation power source20 that is controlled by a power source controller 19. The crucible 1 ais made of platinum, and an induction heating is possible by an inducedcurrent caused by the RF coil 21.

A material support rod 2 c that fixes a seed crystal to an edge isinstalled in an upper part of the crucible 1 a. A support rod liftingmotor 2 a is supplied the electric power from a support rod power source2 b that is controlled by the controller 9, and drives the materialsupport rod 2 c.

The crystallization apparatus according to the present invention furtherincludes the gas introducing port 11 that introduces the gas in thecrucible 1 a. The gas introducing port 11 is supplied a predeterminedgas from the gas tank 16 through the gas valve 14, the valve controller17, the gas purification apparatus 13, and the pipe 12 etc. The gasintroducing port 11 has a movable (up and down) structure to insert intothe melt liquid in the crucible 1 a to introduce the gas after thecrystalloid material is melted, and to shelter from the crucible 1 aduring the crystal growth. The gas introducing port 11 is made of a samematerial as the crucible 1 a so as not to pollute the melt liquid.

Powdery lithium tantalate (LT) is put in the crucible 1 a, the chamber 6is vacuum-exhausted, and the crucible 1 a is heated up to 1670° C. ofthe melting point, and the material in the crucible 1 a is melted. Then,the vacuum exhaust is stopped, and the chamber 6 is returned to theatmospheric pressure by inserting the gas introducing port 11 in thecrucible 1 a, and introducing oxygen at the rate of 200 sccm. Inaddition, oxygen is introduced for three hours while the chamber 6maintained to the atmospheric pressure.

Then, the atmospheric pressure is maintained for one hour after theintroduction of oxygen is stopped, the material support rod 2 c thatfixes the seed crystal to the edge is moved so that the seed crystalcontacts the melt liquid, and the crystal growth is begun. The crystalgrowth is done while lifting the material support rod 2 c at speed of 1mm/h and rotating the material support rod 2 c at 40 rpm.

Table 2 shows a transmittance (wavelength: 300 nm) and a density ofmicro-bubbles of the lithium tantalate crystal obtained by the instantembodiment, that compared with a lithium tantalate crystal grown underother conditions. The comparison examples are an example of introducingthe helium gas instead of oxygen and an example of growing the crystalwithout introducing gas under the oxygen atmosphere. TABLE 2 DENSITY OFMICRO- BUBBLE TRANSMITTANCE (%) (piece/L) LT CRYSTAL OF THE INSTANT 655.2 EMBODIMENT LT CRYSTAL INTRODUCED He 52 6.7 LT CRYSTAL WITHOUT 4527.8 INTRODUCING GAS

Reference to Table 2, the transmittance has improved and the density ofmicro-bubbles has decreased when the oxygen is introduced (the instantembodiment). On the other hand, when the helium gas is introduced, thetransmittance is not improved enough even though the decrease in thedensity of the micro-bubbles is achieved. When the helium gas isintroduced, a part of the oxygen in the crystal material melts into themelt liquid, and the oxygen defect is taken into the grown crystal eventhough the gas is removed.

The instant embodiment explains the crystal growth of the lithiumtantalate crystal, but the same effect can be achieved with the lithiumniobic acid crystal that has a same structure.

The lithium tantalate crystal and the lithium niobic acid crystalmanufactured by the above method has a composition near a stoichiometriccomposition has excellent transmittance, and therefore, can be used as awavelength sensing element, an optical modulator, an optical switchelement, and a digital hologram memory element besides for just a lasermedium.

This application claims foreign priority benefits based on JapanesePatent Application No. 2004-110836, filed on Apr. 5, 2004, which ishereby incorporated by reference herein in its entirety as if fully setforth herein.

1. A crystallization method comprising the steps of: melting acrystallized material in a crucible by heating; and growing a crystal bycooling and coagulating the melted material, wherein said melting stepincludes introducing a predetermined gas into the melted material.
 2. Acrystallization method according to claim 1, wherein said crystallizedmaterial is a calcium fluoride with an added scavenger, and the gas thatintroduced into the melted fluorite is an inert gas.
 3. Acrystallization method according to claim 2, wherein said inert gas is ahelium gas.
 4. A crystallization method according to claim 2, whereinsaid crystallized material is a single crystal that does not have agrain boundary.
 5. An optical element made of a single crystal, saidsingle crystal being manufactured by a crystallization method thatincludes: melting a crystallized material in a crucible by heating; andgrowing a crystal by cooling and coagulating the melted material,wherein said melting step includes introducing a predetermined gas intothe melted material, and wherein said crystallized material is a calciumfluoride with an added scavenger, and the gas that introduced into themelted fluorite is an inert gas.
 6. An exposure apparatus comprising: alight source; an illumination optical system for guiding a light fromthe light source to a reticle; and a projection optical system forguiding the light from the reticle to a wafer that is placed on a waferstage, wherein said illumination optical system or the projectionoptical system includes an optical element, and wherein said opticalelement is made of a single crystal, said single crystal beingmanufactured by a crystallization method that includes: melting acrystallized material in a crucible by heating; and growing a crystal bycooling and coagulating the melted material, wherein said melting stepincludes introducing a predetermined gas into the melted material, andwherein said crystallized material is a calcium fluoride with an addedscavenger, and the gas that introduced into the melted fluorite is aninert gas.
 7. A crystallization method according to claim 1, whereinsaid crystallized material is a lithium tantalate crystal or a lithiumniobic acid, and the gas introduced into the melted lithium tantalatecrystal or the lithium niobic acid is an oxygen or inert gas.
 8. Acrystallization method according to claim 7, wherein said inert gas is ahelium gas.
 9. A crystallization apparatus comprising: a cruciblearranged in a predetermined temperature distribution, and houses amelted crystallized material; and a gas introducing part for introducinga gas into the melted crystallized material, wherein the crystallizationapparatus coagulates the melted crystallized material in one directionby changing the temperature distribution relatively to the crucible. 10.A crystallization apparatus comprising: a crucible heated at apredetermined temperature which houses a melted crystallized material;and a gas introducing part for introducing a gas into the meltedcrystallized material, wherein the crystallization apparatus grows acrystal by lifting a member that contacts with the melted crystallizedmaterial.
 11. A crystallization apparatus according to claims 9 or 10,wherein the gas is an inert gas.